Inertia sensors are mounted in vehicles to sense an inertia condition of a predetermined force or G level and to operate an electrical circuit for controlling operation of the seat belt retractor mechanism. A typical inertia sensor proposed heretofore comprises a rolling ball which rolls up a metal inclined surface to engage another metal surface with current then flowing through the ball and between the two metal surfaces to complete an electrical circuit. Such balls may be noisy in their travel and may be subject to malfunction if liquid or other material is able to coat the contacting surfaces. Other inertia sensors heretofore proposed, use an inertia weight such as a pendulum which swings at a predetermined acceleration or deceleration force to make or break electrical contacts. Electrical contacts are subject to burning or otherwise becoming corroded particularly in vehicles exposed to inclement or corrosive atmospheres. In addition to being reliable and having a long life, inertia sensors for use with seat belt retractor apparatus should also be relatively simple and sufficiently low in cost to compete with existing inertia sensors.